The invention relates to a scanning device for optically scanning an information plane, which device comprises a radiation source supplying a scanning beam and a mirror objective for focusing the scanning beam to a scanning spot in the information plane. The invention also relates to a mirror objective suitable for use in this scanning device and to a write and/or read apparatus provided with such a device.
Scanning an information plane is to be understood to mean both scanning for reading a prerecorded information plane and scanning for writing information in this plane by means of a radiation beam which is intensity-modulated in accordance with the information to be recorded. If a magneto-optical record carrier is recorded, the radiation beam may also have a constant intensity and the magnetic field may be modulated in accordance with this information to be recorded. The information plane may be a plane of an optical record carrier, but also a surface or a plane in an object to be examined, the scanning device then forming part of, for example a microscope.
A scanning device as described in the opening paragraph, intended for reading an optical record carrier, is known from British Patent Specification No. 1,541,596. This specification describes an optical scanning device which includes an objective comprising two mirrors whose reflective sides face each other. The one mirror is concave and its reflective side faces the record carrier. The other mirror, which is much smaller, is convex and its reflective side faces the radiation source. The radiation from the radiation source reaches the convex mirror via a passage in the larger concave mirror. The radiation is subsequently reflected towards the concave mirror and focused by this mirror to a radiation spot in the information plane of the record carrier.
The known objective forms part of a two-stage focusing system in which the entire mirror system is suspended in a magnetic coil with which the position of the objective with respect to the record carrier is controlled. The small mirror is mounted on a piece of piezoelectric material and can be moved up and down with the aid thereof so that a small extent of defocus is periodically produced in the information plane, which defocus is detected by a detection device and from which subsequently a focus-error signal is derived which is used for re-adjusting the position of the objective by means of the magnetic coil.
By using two separate mirrors in a close working relationship with each other, the known scanning device has a rather complicated structure and is sensitive to mechanical perturbations.